This invention relates in general to strain gage transducers for measuring forces in which an electrical resistance type strain gage is affixed to the surface of an elastic member or beam which is subjected to a force. The strain in the member or beam due to the applied force changes the electrical resistance of the gage in proportion to the strain and the force can thereby be measured electrically.
Such gages came into use in World War II and have been steadily improved since then. They are so convenient to use, the gage itself is so small, and the ready use of the output of the gages in electrical circuits and consequent ease of conversion to digital form for data processing, has led to an ever increasing demand for their use and for their improved accuracy.
A typical use for such transducers is in weighing materials. Of course, gravimetric scales of the balance beam type have been used for centuries for weighing, whenever accuracy is required, whereas spring balance weighing devices have traditionally been considered relatively crude and inaccurate. Strain gage transducers, of course, fall into the general category of spring balances, and all of the problems which are inherent in the conventional spring balance context, such as sensitivity to changes in temperature, fatigue, aging, creep, etc. are present also in the strain gage context.
Many of these problems however have been largely solved. For instance, very effective methods for compensating for temperature change have been worked out for strain gages. Other problems such as creep compensation, relaxation compensation and humidity variation compensation have also been solved or largely reduced.
One of the primary problems, however, still requiring solution is the point-of-load sensitivity problem, particularly when vertical forces only are to be measured. It will be understood, of course, that whenever the context permits the weighing platform to be supported by a parallelogram cantilever configuration the point-of-load sensitivity problem can be virtually eliminated. This configuration has been used primarily with low value force strain gage transducers, very effectively (see e.g. U.S. Pat. No. 2,866,059 and 3,439,761). The parallelogram configuration of these cited patents permits the vertical force component to be measured independently of the variations in the point-of-load, provided the gages are very accurately located so as to measure the strain at two or more locations on the parallelogram configuration. In fact even greater accuracy can be attained by compensating electrically for minor discrepancies in the location of the gages (see e.g. U.S. Pat. No. 3,576,128). In the parallelogram configuration the strain gages are actually mounted so as to measure the bending strain at two or more locations on the parallelogram but this does not mean that the bending moment of the overall system is being measured. In fact, the manner in which the outputs of the strain gages are used, cancels out the effect of the bending moment and gives a measurement strictly of the vertical (shear) forces acting on the transducer. The principal advantage of the properly gaged parallelogram type configuration is that it permits the vertical forces to be measured independently of variations in the point-of-load.
The parallelogram configuration, however, although good for lighter weights, is not suitable for measuring heavy weights (e.g. 10,000 lbs. and greater). Obviously, a massive parallelogram element might be devised which would have low point-of-load sensitivity for such large forces, but it would be undesirably bulky for many uses.
For the measurement of larger forces, transducers having gages positioned to measure the vertical forces directly in terms of shear only have been used with only moderate success. Shear forces are normally measured by aligning the strain gages so as to measure both the tensile or compressive principal strains. Usually for shear measurements, the gages are mounted at 45.degree. and 135.degree. from the longitudinal axis of the beam of which shear force measurement is to be taken. However, at such an angle the shear force measuring gages will also measure bending strain unless they are accurately located at the neutral bending axis because bending stress acts horizontally in tension above the neutral axis of the beam and horizontally in compression below the neutral axis. Placing the shear force measuring gage on the neutral axis eliminates this because when the gage is so positioned, one half of the gage lies in the upper (tensile) area of the bending stress, and the other half lies in the lower (compressive) area of the bending stress so that the effects of bending stress on the shear measurement will theoretically cancel out. Another way to achieve the same result is to use two compressive and two tensile shear force gages and to place one tensile gage and one compressive gage above the neutral axis and the other tensile gage and the other compression gage a similar distance below the neutral axis. With such an arrangement, theoretically, the influence on the gages of the bending strains can be cancelled out.
A prior art patent in which both of the latter methods for isolating the measurement of shear force stress from bending stress is U.S. Pat. No. 3,554,025. In addition, U.S. Pat. No. 3,554,025 also discloses two other ways to eliminate or minimize the effect of bending stress. One is to provide a double beam which has a base beam and a second beam connected to the free end of the base beam and extending back over the base beam. With this arrangement the gages are affixed to the base beam at a point where they are in line with (or balanced to each side of) the thrust axis of the load. In this way the gages sense only shear related strains, and when the point-of-load position changes, the change of lever arm which causes a change in bending moment which would distort the measurement of shear, is so small that its influence is negligible. Further improvements can be made by selecting a cross-sectional geometry in which there are locations at which the strain gages can be applied where bending strain is minimized and shear strain is maximized. Typically, this is done with an I-beam configuration. With an I-beam, the shear strain is highest and relatively uniform in the vertical web area, and therefore the I-beam configuration is recommended in U.S. Pat. No. 3,554,025 for the specific purpose of maximizing shear related stresses at a location where the bending related stresses are relatively low. Of course, this is done for the basic purpose of maximizing the shear strain measurement and thereby rendering the transducer less sensitive to changes in the point-of-load.
It is a fact, however, that the foregoing measures have not achieved their objective. The double (or triple) beam arrangement of U.S. Pat. No. 3,554,025 is not adequate for very heavy loads unless it is constructed with prohibitively massive components. In addition, the use of techniques such as the careful positioning of the gages, and the selection of special cross-sections have not been altogether successful. In a typical conventional arrangement employing a straight cantilever beam, the transducers usually have no greater point-of-load sensitivity than 0.3% per one-fourth inch of displacement of the load. At first glance, one might regard such an insensitivity as being rather good, but in practice, with massive loads upwards of 10,000 lbs., it is virtually impossible to reduce the unavoidable changes in point-of-load to less than one-half inch. This is due largely to changes in the dimensions of the load platforms due both to thermal expansion and loading variations. Where extremely heavy forces are involved, since knife edge pivots are impractical, arcuate supports are employed, but with such supports, when the platform (or transducer) bends, an automatic change in the "point-of-load" application occurs. In many cases the change exceeds one-half inch. Such a change, however, with the conventional transducer, would represent a 0.6% error, and in the bulk-materials industry, would be prohibitively large. Government regulations require greater accuracy.
Prior to the present invention, no strain gage transducer solution to the point-of-load sensitivity problem for very heavy weights, better than 0.3% per one-fourth inch of motion, has been available. It would appear that all prior attempts to eliminate the effects of bending stress, and to employ geometries selected in order to minimize bending strain, have not succeeded in eliminating or sufficiently reducing a certain inevitable and hitherto not fully appreciated interaction between the bending and the shear strains which distort the shear force measurements. The result has been that point-of-load sensitivity has remained a major problem.